1. Field of the Invention
The present invention relates to an endoscope system that selectively applies normal light such as white light and special light such as infrared light to an internal body part, and a drive control method thereof.
2. Description Related to the Prior Art
A medical examination using an electronic endoscope is widely available in recent years. The electronic endoscope has a solid-state image sensor such as CCD at a distal portion of an insert section to be introduced into a human body cavity. The electronic endoscope is connected to a processor device and a light source device through a cord and a connector.
The processor device applies signal processing to a pickup signal outputted from the solid-state image sensor to produce an endoscope image. The endoscope image is displayed on a monitor connected to the processor device. The light source device has a white light source such as a xenon lamp. The light source device supplies the electronic endoscope with the white light to illuminate an internal body part.
In the field of medical diagnosis using the electronic endoscope, a method called Narrow Band Imaging (hereinafter abbreviated as NBI) sits in the limelight. In the NBI, light with a narrow wavelength band (hereinafter called special light) is applied to the internal body part, instead of the white light (hereinafter called normal light), which has broad spectral characteristic in a visible light range, and reflection of the special light forms an image. An image taken under illumination of the normal light will be called as a normal light image, and an image under illumination of the special light will be called a special light image. The NBI allows easy obtainment of an image that emphasizes a blood vessel beneath the mucous membrane and an image that emphasizes components of an organ such as a stomach wall and intestine cortex tissue, and hence facilitates finding out a lesion.
Japanese Patent Laid-Open Publication No. 2002-095635 or U.S. Pat. No. 5,667,474 (corresponding to Japanese Patent Laid-Open Publication No. 07-246184) proposes an endoscope system adopting the NBI. In such an endoscope system, a filter that integrally includes a normal light filtering section and a special light filtering section is disposed in an optical path of light from a light source. The filter is mechanically moved by a motor or the like in response to switching operation of an operator (doctor), so as to obtain both of the normal and special light images.
An endoscope system according to Japanese Patent Laid-Open Publication No. 2002-095635 (corresponding to U.S. Patent Application Publication No. 2003/0176768) is provided with a rotational filter that selectively outputs the normal light and special light. Switching from the normal light to the special light reduces light intensity. Accordingly, it is described in paragraph [0043] to extend exposure time in special light observation. Extending the exposure time, however, increases fluctuation in the image. Thus, the endoscope system is provided with a movement detection circuit for detecting movement of an object. When it is judged that there is no movement during freeze operation, an image stored on an image memory is updated. When there is movement, the image is not updated. In sixth and seventh embodiments described in paragraphs [0089] to [0106], a xenon lamp is used as a normal light source, and a super-high pressure mercury lamp is used as a special light source.
U.S. Pat. No. 5,667,474 discloses a field sequential image pickup apparatus that is provided with a filter and a movement detection circuit as with above. In this apparatus, when it is judged that an object has moved during normal light observation, freeze operation is inhibited. When it is judged that there is movement during special light observation, an adder suspends adding operation (noise reduction operation). When it is judged that there is no movement, the adder resumes the noise reduction operation.
Japanese Patent Laid-Open Publication No. 06-276529 also discloses an image pickup device having a movement detection circuit. In this device, the degree of contour emphasis varies in accordance with the degree of movement of an object. In other words, the slower the object moves, the more a contour signal is emphasized.
In the NBI, it is demanded to display the normal light image and the special light image simultaneously on a single screen for the purpose of making diagnosis with comparing both images in detail. However, there is a relatively large gap between the timing of taking the two images. If an object moves during an interval between the two images, simultaneity (equality) is not maintained. Accordingly, the doctor cannot carry out precise comparison and make correct diagnosis.
In Japanese Patent Laid-Open Publication No. 2002-095635 and U.S. Pat. No. 5,667,474, the filter is mechanically moved, and the normal light image and special light image are separately displayed. The normal and special light images are selectively switchable, but cannot be displayed together. In addition, neither of the systems intends to maintain the simultaneity of the images. Relative movement of the object is detected, but detection result is used only for judging whether or not to update image data during the freeze operation, or whether or not to carry out the freeze operation and noise reduction operation. None of the foregoing documents considers simultaneity in displaying the normal and special light images on the single screen.